This invention relates to X-ray beam position monitors.
X-ray diffractometers and cameras, especially those intended for macromolecular crystallography, are relatively large and heavy. The problem of aligning such instruments to the X-ray beam from a rotating anode generator requires very sturdy yet precise translational adjustments. Recently, compact sealed micro-focus X-ray tubes have been developed with a magnetically focused and steered electron beam (Arndt, Long and Duncumb, 1998). These tubes make it possible to keep the diffractometer stationary and to move the X-ray beam into alignment. In most of our applications the tube is used with focusing mirrors (Arndt et al., 1998) which are first aligned with respect to the tube; this alignment can be carried out with small and compact translations or, indeed, can be effected entirely by magnetically deflecting the electron beam on the X-ray tube target. The assembly of tube and collimators can then be moved into position relative to the diffractometer by relatively light translational adjustments. In those applications where pin-hole collimation without focusing mirrors is used, mechanical movements can be dispensed with entirely and the X-ray beam moved using the magnetic deflection of the focal spot on the tube target only.
In both applications alignment is greatly helped by an X-ray beam position monitor, preferably by one which has an electrical output which can be used for controlling the translations so as to make alignment automatic. We have found the split ion chamber (Koyama, Sasaki and Ishikawa, 1989; Schildkamp and Pradervand, 1995; Billing, 1998) ideal for our purposes. Beam-position monitors such as split-plate ion chambers are commonly employed in synchrotron radiation beam lines (Alkire, Rosenbaum and Evans, 1999). However, for laboratory sources and diffractometers compactness is of paramount importance and we have constructed a chamber which allows position monitoring in two orthogonal planes.
According to the invention an X-ray beam position monitor comprises a first plate assembly for detecting the position of the X-ray beam in one plane, the first plate assembly comprising a first pair of collection plates and a first biasing plate, a second plate assembly for detecting the position of the X-ray beam in another plane transverse to said one plane, the second plate assembly comprising a second pair of collection plates and a second biasing plate, switching means for applying a bias voltage to the first biasing plate or the second biasing plate and signal processing means for processing electrical signals which are generated at the collection plates and which are representative of the position of the X-ray beam, wherein the first plate assembly and the second plate assembly are positioned at the same, or substantially the same, axial position along the direction of propagation of the X-ray beam. The placement of the first plate assembly and the second plate assembly at substantially the same position along the direction of propagation of the X-ray beam provides a compact arrangement which renders the monitor particularly useful for use with X-ray diffractometers and for laboratory use generally. This contrasts with the arrangements disclosed in Schildkamp and Pradervand where the X-ray beam passes first between a first plate assembly and then, afterwards, between a second plate assembly.
The plates of the first assembly are preferably orthogonal to the plates of the second assembly, so that said one plane and said another plane are mutually orthogonal. In this case, the first and second plate assemblies preferably constitute the four walls of a square-section tunnel-like structure through which the X-ray beam is propagated.
The X-ray beam position sensor preferably acts as a null-seeking device, the beam being centred by means of successive adjustments in the two planes of positioning, until the electrical signals are representative of a centred position of the X-ray beam.
An X-ray beam position sensor according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: